It has been known in the art to add stable complex salts of transition metals such as zinc to emulsions and dispersions of acid containing polymers, (U.S. Pat. Nos. 3,308,078, 3,328,325, 3,467,610, 3,554,790, 4,150,005, and 4,517,330, ).
In practicing this chemistry, complex salts are formed from simple salt or oxides of transition metals with amines or other simple complexing ligands. (In the following formulae `M` indicates a transition metal, `L` indicates a ligand). ##STR1##
Since each of the above steps in the formation of the complex from the free (or hydrated) metal ion is reversible and runs to equilibrium, the process must be forced to completion (tetradentate ligand complex) by mass action, charging an excess of the ligand species. The complexing agent must be a simple ligand, to avoid the formation of very stable complex structures that will not donate metals to the acidic polymer.
The metal complex is formed before addition to the polymer to increase the ion complex surface area, decreasing the charge per unit area, so that the acid containing polymer is stable in the presence of the multivalent ion. The instability of acid containing polymers to multivalent ions is well known and, in fact, they be are commonly used to flocculate and precipitate polymers from waste streams (Fe++, Fe+++ and AI+++ salts are most commonly used). The reduced charge density of the complex multivalent salt provides only minimal disruption of the polar double layer thought to be responsible for polymer emulsion stability.
When the complex salt solution is added to the acidic emulsion polymer, the salt undergoes counterion exchange. Most commonly, the complex multivalent cations are prepared as carbonate, bicarbonate, or acetate salts. As this technology is generally understood, the only limitation of the anion of the salt is that it be a stronger base than the anion of the pendant polymeric acid. If weaker base anions, such as chloride, etc., are used as the salt, crosslinking apparently does not occur because the process of counterion exchange does not happen; the weaker base anions do not displace the anion of the polymeric acid. (In the following ##STR2## represents an acid functional group attached to a polymer). ##STR3##
The conjugate acid of the anion of the stable metal complex must be either volatile or unstable. For instance, acetic acid, the conjugate acid of acetate anion, is volatile, and carbonic acid, the conjugate acid of both bicarbonate and carbonate anions, is unstable (spontaneously decomposing to carbon dioxide and water). In practice, the evolution of volatile conjugate acid, or the volatile by-products of the decomposition of the unstable conjugate acid is a processing problem encountered duringthis crosslinking reaction.
The complex cation, in close association with polymer carboxylate anions provides latent crosslinking of the polymer (Maintenance Chemical Specialties, by Walter J. Hackett. Chemical Publishing Co., Inc. N.Y., 1972. pp9-13. ). This crosslinking has been referred to as latent because it occurs only after the volatile (amine) ligand is released from the metal during the polymer film formation stages. ##STR4##
The latent crosslinking may be due to the formation of insoluble metal-polymeric carboxylate salts, or the formation of polymeric carboxyl complexes with the metals. ##STR5##
Complexed transition metal salt latent crosslinking has thus enabled the art to produce polymers that will crosslink in a coating upon drying, without interfering with the film formation process. Since the final crosslinked polymer effectively has the pendant acid functionality tied up in insoluble acid-metal salts or complexes, metal crosslinked polymers have improved resistance to alkaline materials, such as detergents or cleaning solutions.
The addition of low levels (typically 1 to 3%) of ammonia or other amine to a cleaner solution is believed to effectively reverse the crosslinking process. The free metal-amine complex is re-formed, thus freeing the polymeric acid functionality which may then be attacked by simple alkaline materials. These amine-containing cleaner solutions are known as strippers, since they effectively allow for the removal of the previously crosslinked films.
One problem of this chemistry has been that application of multiple coats of compositions containing these metal salt complexes can sometimes prove difficult because the new wet coat of polymer composition contains a high concentration of the complexing amine liqand. This high concentration of free amine, and the amine ligand released from the complex, act as a stripper on the previously applied under-coat causing redispersion of the under-coat, drag in the application of the top coat, whitening and ghosting of the coating, and general disruption of the recoating process known as poor recoatability. These difficulties are particularly noted when coating formulations are applied rapidly, as is common practice in industrial applications.
Though transition metal salt latent crosslinking of acid-containing emulsion polymers has provided many improvements in dry film properties, the high ammonia content of transition metal complex formulations is disadvantageous in that it is mildly toxic and highly odoriferous. The volatile ligands lead to difficulties in handling, formulating, and use of the emulsion polymers produced by this technology.